There are many physiological and pathological conditions of animal tissue where the supply of exogenous ribonucleosides may have useful therapeutic applications. In a number of physiological and pathological conditions, the administration to an animal of RNA, nucleotides, or individual or mixtures of nucleosides, has been shown to improve the natural repair processes of the affected cells.
There are many important metabolic reactions that are usually functionally subsaturated and limited by availability of either substrates or cofactors. Such rate-limiting compounds may be either nutritionally essential or synthesized de novo in the body. Under conditions of tissue trauma, infection or adaptation to physiological demand, particularly when cellular repair or regeneration processes are activated, the optimum nutritional, biochemical, or hormonal environment for promoting such repair may be quite different from the requirements for normal cell and tissue function. In such cases, therapeutic benefit may be derived by providing appropriate conditionally essential nutrients, such as ribonucleosides or metabolites which may be required in quantities not usually available from a normal diet.
The therapeutic potential for this strategy of directly supporting the metabolic function of damaged or diseased tissues has not been realized in contemporary medical practice. For example, the routine treatment of cardiac insufficiency involves the use of agents which elicit increases in cardiac work output (e.g., cardiac glycosides), but which do not support or improve the actual functional capacity of myocardial metabolism. Similarly, in the cases of liver disease or damage, cerebrovascular disorders, bone fractures, respiratory distress syndrome, senile dementias, and most other clinical conditions, contemporary treatments generally do not include intentional methods for improving the capacity of tissues to undergo repair or compensatory metabolic or structural adaptations.
At the cellular level of organization, there are specific metabolic responses to trauma that are involved, in a variety of tissues, in the processes of tissue repair, regeneration, or adaptation to altered functional demand. Most processes of tissue damage and repair are accompanied by a substantial increase in the activity of the hexose monophosphate pathway of glucose metabolism.
The hexose monophosphate pathway is the route of formation for the penrose sugars (e.g., ribose) which are necessary for nucleotide and nucleic acid synthesis. The availability of ribose is rate limiting for nucleotide synthesis under most physiological or pathological conditions. Rapid production of nucleotides for the synthesis of nucleic acids and nucleotide-derived cofactors (such as cytidine di-phosphocholine or uridine di-phosphoglucose (UDPG)) is essential for the processes of tissue repair and cellular proliferation. Even though nucleotides are synthesized de novo from simpler nutrients, so that there is not an absolute dietary requirement for direct nucleotide precursors, many tissues may not have optimal capacity for nucleotide synthesis particularly during tissue repair or cellular proliferation.
It is possible to bypass the limited capacity of the hexose monophosphate pathway by providing preformed ribonucleosides directly to tissues where they are incorporated in the nucleotide pools via the "salvage" pathways of nucleotide synthesis. It is also possible that pyrimidine ribonucleosides may exert therapeutic influences through mechanisms unrelated to the support of nucleotide biosynthesis.
The effects of the administration of pyrimidine nucleosides, and in particular, uridine and cytidine, on a variety of physiological and pathological conditions in experimental animals and to some extent in humans have been extensively studied. These are summarized below.